Application of a sensitive test (the Clq binding method) for the detection of soluble antigen-antibody complexes to sera from patients with a variety of tumors has shown that over 80% of 134 patients with cancer probably had circulating immune complexes. The quantity of these was inversely proportional to the duration of the disease and patients who had had cancer less than 5 months and who had evident disease remaining after surgery had the highest levels. Moreover, in following these patients for up to 7 months we have found that patients with high levels of Clq binding activity (greater tha 10%) tend to have a significantly higher rate of progression on therapy than do patients with lower Clq binding activities. Thus, measurement of circulating immune complexes in sera may provide an estimate of residual tumor mass in a variety of tumors for which specific immunodiagnostic reagents such as carcinoembryonic antigen and its antibody are not available. Moreover, measurement of soluble complexes provides a means to identify sera rich in both tumor antigen and antibody so that these may be isolated, fractionated and purified. It is proposed that serial measurements of soluble complexes be studied in a wide variety of malignant diseases over a 5 year period to determine the value of the method in predicting both remission and progression of disease. It is also proposed that the complexes be used as a raw material for characterization and purification of tumor antigens to create new reagents for diagnostic purposes and also to investigate the physiological role of these complexes in the tumor-host interaction.